In analysis technology or sensor technology, potentiometric measurements are performed, i.e., electrical potentials are measured without impressing a current into the measurement object. Said electrical potentials represent the property of the analyte to be characterized or are generated from the property of an analyte to be characterized by means of a so-called transducer. A sensor for this converts said electrical potentials into electrical signals, for example voltage signals, which are subsequently evaluated. By way of example, in pure medical research, electrodes are used during the potentiometric measurement of extracellular neural signals.
In accordance with K. Takahashi, S. Takeuchi, Bi-MOSFET Amplifier for Integration with Multimicroelectrode Array for Extracellular Neuronal Recording, IEICE TRANS. FUNDAMENTALS, Vol. E77-A, No. 2, 388–393, February 1994, an electrode module is provided for measuring electrical potentials of neurons. The electrode module produced from semiconductor material contains a tapering pointed microelectrode and, at its wide end, a rectangular substrate with an amplifier thereon.
One problem with the electrode module according to K. Takahashi et al. is that it is not possible to simultaneously measure neuron assemblages in a spatially resolved manner, since the tapering pointed microelectrode can only perform point measurements. On account of this, in the case of the present electrode module, the dimensioning of the amplifier is also of secondary importance since no structural space constraints whatsoever result.
P. Fromherz, Interfacing Neurons and Silicon by Electrical Induction, Ber. Bunsenges, Phys. Chem. 100, No. 7, 1093–1102, 1996, discloses a field-effect transistor used as a sensor and serving for electronically reading out neural signals of living cells. In this case, a living nerve cell bears on the insulated, open gate of the field-effect transistor. This sensor can record neural signals of the cell, which are manifested in the form of changes in potential at the cell wall, since these changes in potential control or modulate the channel current of the transistor or the density of the charge carriers present in the channel region between drain and source.
The arrangement disclosed in P. Fromherz was extended in A. Stett et al., Two-way silicon-neuron interface by electrical induction, Physical Review E, Volume 55, Number 2, 1779–1782, February 1997, to the effect that, by means of bidirectional connection, the potential of a nerve cell can be altered by the application of an electrical signal to the arrangement. For this purpose, a control electrode is arranged beside the FET structure in a manner insulated from the latter, which control electrode, by the application of a corresponding electrical signal, causes the nerve cell to change its electrical potential.
S. Vassanelli, P. Fromherz, Transistor records of excitable neurons from rat brain, Appl. Phys. A66 459–463, 1998, shows the previously described arrangements in the application when determining potentials of nerve cells of a rat.
In the case of an external signal gain connected to the above-described sensor, on account of the small signal amplitudes, a poor signal transfer, a poor signal-to-noise ratio, and also a higher sensitivity toward disturbing influences (e.g., leakage fields) and parasitic effects are disadvantageous.
W. J. Parak et al., The field-effect-addressable potentiometric sensor/stimulator (FAPS)— a new concept for a surface potential sensor and stimulator with spatial resolution, Sensors and Actuators, B-58, 497–504, 1999, discloses an array of ion-sensitive field-effect transistors which are constructed on a GaAs substrate and are arranged in rows and columns. In order that the individual transistors can be arranged as closely adjacent as possible, the gates of a FETs arranged in a common column were connected to one another to form a back gate, as were the channels of FETs located in a row. By sequentially tapping columns and rows, it is possible to determine the location of a change in potential within the measuring array.
What is disadvantageous about the arrangement according to W. J. Parak et al. is that the current path has to flow via a series circuit of sensors. As a result, it is not possible to realize large arrays since the series resistance with regard to the selected sensor leads to an attenuation of the signal in accordance with a voltage divider.
Furthermore, a very complicated production process is necessary in order to realize a back gate. Furthermore, a special GaAs technology is used, the material arsenic which is used being highly toxic and the sensor thus only being suitable for a small proportion of possible applications.
T. C. W. Yeow et al., Design of a Single Cell of an ISFET Array Chip, Microelectronics: Technology today for the future, NA (NA) 62–67, 1995, shows an array of ion-sensitive field-effect transistors on a common substrate. In this case, the array does not serve to enable individual transistors to detect local impedances in their surroundings which differ from the impedance acting on the adjacent transistor. Thus, a highest possible resolution of the array is not desired at all. Instead, precisely the measurement sample is to have constant properties across the array, because the array serves as an analog-to-digital sensor: in this case, a threshold value is assigned to each sensor cell designed as a field-effect transistor. The threshold values differ from sensor cell to sensor cell. A comparison circuit checks whether the measurement quantity exceeds the respective threshold value. Thus, an array of binary values which reproduces a specific measured value is produced with regard to a sample covering the array. Read-out circuits are integrated on the substrate.
T. C. W. Yeow et al., A very large integrated pH-ISFET sensor array chip compatible with standard CMOS process, Sensors and Actuators B 44, 434–440, 1997, shows an arrangement according to T. C. W. Yeow et al. developed further. In this case, each sensor row is assigned only one comparator, the threshold values of which can be altered, so that the individual sensors are read sequentially.
T. C. W. Yeow et al., Thick-Film Thermistor Array Using a Novel Threshold Conversion Concept, Sensors and Materials, Vol. 10, No. 2, 77–91, 1998, shows further mechanisms for threshold value evaluation in this regard.
In the case of the arrangements described last, the output values obtained in this way in the individual sensors are summed and averaged. This average value then depends linearly on the measurement quantity.
However, this only enables relatively slow measurements without spatial resolution, since, on the one hand, the entire array has to be covered by the sample in order that all the sensors can record the same quantity and compare it with the assigned threshold values. On the other hand, the read-out mechanisms are complicated and slow, so that the arrangement is only suitable for static measurements.
By contrast, a very high packing density of sensors is necessary for the spatially resolved measurement of, for instance, very small extracellular signals of individual neurons within an assemblage of neurons.
In order to be able to measure the signals of individual neurons, the output signals of the sensors of the array must be able to be read out individually. The distance between adjacent measuring cells—also called pixels hereinafter—which in each case contain the sensor circuits must be smaller than the neurons themselves. However, on the one hand, minimum feature sizes dictated by the production process represent a boundary condition for the production of sensors. On the other hand, statistical fluctuations in the parameters and the noise proportions of the sensor signals increase greatly as the area of the components decreases.
Therefore, in many applications, the components of the sensor circuits have to be given larger than minimum dimensioning in order to guarantee an acceptable signal-to-noise ratio. Thus, pixel circuits have to be realized with a smallest possible number of components in order to achieve a small pitch and good signal-to-noise ratios. Furthermore, it is also necessary to ensure stability, high sensitivity and robustness of the arrangement.
Furthermore, EP 0 942 259 A1 describes a capacitive distance sensor for acquiring a fingerprint image. The sensor has a capacitive element with a first and a second capacitor plate, which are arranged next to one another and whose distance with respect to one another is measured. The sensor furthermore has an inverting operational amplifier, in whose feedback path the capacitive elements are connected.
U.S. Pat. No. 5,309,085 describes a measuring circuit with a biosensor, in which ion-sensitive field-effect transistors are used.
U.S. Pat. No. 5,965,452 discloses a multiplexed active biological electrode array.
O. Limann, Elektronik ohne Ballast: Einführung in die Schaltungstechnik der industriellen Elektronik [Electronics with no ballast: Introduction to the circuit technology of industrial electronics], Franzis-Verlag, 7th edition, pp. 35–38, 1987, describes basic circuits with field-effect transistors.